The present invention relates to a motor control method and apparatus, and more particularly to a method of and an apparatus for controlling a secondary scanning motor employed in a radiation image information readout device, a radiation image information recording device, a facsimile machine, or the like for reading or recording image and character information.
There have been developed radiation image information readout devices for reading radiation image information recorded on an image information carrier such as a stimulable phosphor sheet.
When a certain phosphor is exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, ultraviolet rays, or electron beams, the phosphor stores a part of the energy of the radiation. When the phosphor exposed to the radiation is exposed to stimulating rays such as visible light, the phosphor emits light in proportion to the stored energy of the radiation. The phosphor exhibiting such a property is referred to as a "stimulable phosphor".
The applicant has proposed radiation image readout systems employing a sheet of such a stimulable phosphor. The radiation image of an object such as a human body is recorded on the stimulable phosphor sheet, and then the stimulable phosphor sheet is exposed to stimulating light for emitting light therefrom which is read by a photoelectric transducer as an electric signal that is utilized for various diagnostic purposes. See for example U.S. Pat. Nos. 4,258,264, 4,276,473, and 4,315,318.
In one of the radiation image readout apparatus, the stimulable phosphor sheet is fed by a sheet feeder at a constant speed during which time the sheet is scanned with stimulating light one-dimensionally in a direction normal to the direction of feed of the sheet. The light emitted from the sheet upon exposure to the stimulating light is photoelectrically read to produce radiation image information in the form of an electric signal.
More specifically, the stimulable phosphor sheet is mechanically fed in one direction for secondary scanning thereof, while at the same time a light beam such as a laser beam is swept one-dimensionally over the sheet in the direction perpendicularly to the sheet feeding direction for primary scanning of the sheet. Therefore, the stimulable phosphor sheet is scanned two-dimensionally. The light emitted from the stimulable phosphor sheet is detected on a time series basis by a light detector such as a photomultiplier to produce image information.
For improving the readout accuracy, it has been proposed to make a mechanical improvement for feeding the stimulable phosphor sheet at a constant speed. In addition, it has also been proposed to feed the stimulable phosphor sheet in one direction for reading the radiation image in a pre-reading mode, and then feed the stimulable phosphor sheet in the opposite direction to read the radiation image in a main reading mode. Various readout conditions in the main reading mode can be established on the basis of information attained in the pre-reading mode.
The stimulable phosphor sheet is actually fed on an endless feed belt. Since it is necessary to keep the same laser beam scanning position in the pre-reading and main reading modes, the endless feed belt comprises a perforated endless belt and a suction box is disposed below the endless belt for attracting the stimulable phosphor sheet to the endless belt to maintain the sheet and the endless belt in the same relative position.
The stimulable phosphor sheet is however not attracted throughout its entire length along the direction of feed against the endless belt by the suction box. If the stimulable phosphor sheet were to be attracted fully along its entire length by the suction, the suction box would have to be larger in size and hence could not be unitized for greater compactness, and the suction device coupled to the suction box would have to be of a larger capacity, resulting in an increased cost of manufacture. When the pre-reading mode is finished, the stimulable phosphor sheet should be stopped in a position where at least a portion thereof is attracted by the suction box. If the stimulable phosphor sheet were fed away from such a position, then the endless belt and the stimulable phosphor sheet would not be kept in the same relative position. Any resulting positional deviation would cause an image to be differently scanned in the pre-reading and main reading modes, with the consequence that different contents of information would be read out.
To avoid the above drawback, the motor for driving the endless belt should be controlled so as not to overrun.